In the Muller-Rochow direct synthesis, chloromethane is reacted with silicon in the presence of a copper catalyst and suitable promoters to give methylchlorosilanes. In this process, as high a utilization of silicon as is possible in conjunction with safe and simultaneously flexible operation of the entire plant is required in addition to high productivity, as measured by the amount of silanes formed per unit time and reaction volume. High selectivity, based on the desired product dimethyldichlorosilane, is also desirable. Dimethyldichlorosilane is required, for example, for the preparation of linear polysiloxanes.
The direct synthesis can be carried out batchwise or continuously, the continuous procedure preferably being used in industrial production. The continuous direct synthesis is carried out in fluidized-bed reactors, in which chloromethane is used simultaneously as fluidizing medium and reactant. The silicon required is milled beforehand to a powder with a particle size of up to 700 .mu.m, and is mixed with copper catalysts and promoters to give the catalyst material. This catalyst material is then introduced into the fluidized-bed reactor, and is reacted with the chloromethane at a temperature in the range of 260-350.degree. C. Since the reaction is exothermic, the liberated heat of reaction must be removed via a cooling system.
Unconverted chloromethane, gaseous methylchlorosilanes, catalyst components and finely divided dusts leave the reactor. The coarser fraction of the entrained particles can be separated from the gas stream by means of one or more cyclones (main cyclones) and may be either recycled in whole or in part to the reactor, optionally via intermediate collecting containers, or alternatively may be removed from the system by means of dust collecting containers.
The very fine entrained particles must likewise be separated from the gas stream. This can be effected, for example, by gas filtration and/or by one or more downstream cyclones. High utilization of silicon can be ensured by this procedure. For example, such a system consisting of reactor, main cyclone with recycling and downstream cyclone with dust collecting container is shown in U.S. Pat. No. 4,281.149, FIG. 1. The crude silane is then separated from unconverted chloromethane and distilled. The refined, unconverted chloromethane can be fed back the reactor.
The collected dusts must be at least partially removed from the reaction system, since various secondary elements and slag components which are introduced with the silicon would otherwise accumulate in this product stream. When completely recycled to the reactor, selectivity will be greatly reduced by the catalytic effects of these impurities. Furthermore, finely divided, unreactive components which are introduced with the silicon, such as slags and iron suicides, must be removed from the system in order to achieve as high a productivity as possible. If the very fine dusts were to be completely recycled to the reactor, these dusts would be very rapidly discharged again and thus unnecessarily load the gas purification system: the selectivity and the reactivity would decrease owing to catalytic effects, and the production campaign would have to be shortened as a result.
These effects are described in B. Pachaly, H. Straussberger, and W. Streckel, "From Waste to Valuable Products: Work up of Silicon Metal Byproducts from the Direct Process", SILICON FOR THE CHEMICAL INDUSTRY II; Loen, Norway, Jun. 8-10, 1994; pages 235-239;. The dusts mainly comprise fine silicon particles, reacted silicon particles, slag particles, unreactive silicides, catalyst and promoter components and, on separation, have a temperature which corresponds approximately to the temperature of the contents of the reactor. Owing to the large specific surface area; to the presence of catalysts and promoters required for the direct synthesis; and to their free surface, i.e., one not covered with a protective Si0.sub.2 layer, these dusts are very reactive. The dusts can, for example, further react with the chloromethane present and exothenrin in an uncontrolled manner. In "Recent advances in the direct process", THE CHEMISTRY OF ORGANIC SILICON COMPOUNDS; Volume 2, Part 2, Chapter 26, pages 1581-1597;, it is stated that the highly exothermic direct synthesis over silicon freshly milled in the absence of oxygen begins at 147.degree. C. in the presence of the corresponding catalysts. The exothermic reaction of copper compounds with metallic silicon can also lead to uncontrolled exotherm from temperatures of about 150.degree. C.